mfd: core: Use acpi_find_child_device() for child devices lookup

Use acpi_find_child_device() for child devices lookup in mfd_acpi_add_device()
instead of open coded approach. No functional change intended.

While at it, amend a note comment, since usage of _ADR is found on other
platforms and tables than Intel Galileo Gen 2, in particular USB wired devices
are using it, according to Microsoft specifications for embedded platforms.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>